The present invention relates to radar systems and more particularly to radar systems employing electronically steered, radar antenna arrays.
A dish antenna directs a radar beam in a single fixed direction, and the antenna is mechanically repositioned to change the beam direction. The dish antenna is rotated to produce a 360 degree scanning beam.
An electronic radar antenna produces directional beam control through phase control of the individual antenna radiating elements, without requiring mechanically driven movement of the antenna. Generally, the individual radiating elements are operated in combination so that the collective radiation from the elements forms a beam which scans over a field of observation in accordance with electronic steering control. Normally, each radiating element has a phase delay circuit element connected to it to determine when it radiates and thereby provide the basis upon which the beam is formed and scanned.
Electronic beam-steering antenna arrays can be used in various kinds of radar systems. Thus, these arrays can be used in target acquisition systems, communication systems, pulsed radar systems, continuous wave radar systems, etc.
Conventional phase shift elements include ferrite (iron based) devices, PIN diode or other semiconductor devices, and ferroelectric (ceramic) devices. The ferrite phase shift devices can handle higher power signals, but they impose relatively high antenna array manufacturing costs, they operate unidirectionally with nonreciprocity in the propogation of transmit and receive signals and they are highly susceptible to temperature changes making temperature calibration difficult to achieve.
In general, semiconductor phase shifters are compact but they are normally limited to small signal applications. PIN diodes operate with discrete phase steps which disadvantageously results in a jumping beam as opposed to a smoothly scanning beam.
Ferroelectric phase shifters operate with continuous variability, operate under voltage control with low power consumption, and operate reciprocally for transmitting and receiving signals. Accordingly, ferroelectric phase shifters are highly desirable for use in electronic beam-steering arrays.
Similarly to the case of ferrite phase shifters, ferroelectric phase shifters operate with a strong dependence on environmental factors, including mainly temperature and humidity. As a consequence of this dependence, the control of beam direction through ferroelectric phase shifters is adversely affected by environmental variations including variations in the temperature of each ferroelectric phase shifter element connected to an antenna array.
Automatic temperature calibration has been implemented in beam steering control to compensate for temperature-based errors in radar systems employing ferroelectric phase shifters, as disclosed in U.S. Pat. No. 5,680,141, entitled TEMPERATURE CALIBRATION SYSTEM FOR A FERROELECTIC PHASE SHIFTING ARRAY ANTENNA, filed by the current inventors on May 31, 1995, and assigned to the current assignee. In that prior disclosure, a controller employs a calibration function which represents the relationship between temperature and calibration error factors that are multiplied against basic phase shift control data to produce calibrated phase shift control data for temperature compensated beam steering. The prior calibration function uses an equation which is stored as a tenth order polynomial representing error factor versus temperature for the controlled antenna array.
The referenced prior system thus basically provides temperature-calibrated antenna operation in a radar system employing a ferroelectric phased array antenna. However, the prior system requires excessive computation capacity and makes phase voltage corrections with limited accuracy.
Thus, multiplication procedures emloyed in the referenced prior system is computation intensive thereby limiting the utility of the prior system. More particularly, a polynomial equation converts feedback temperature deviation from a nominal setpoint to a correction factor which is multiplied against the uncorrected, steering drive voltage at the nominal temperature. The product is a corrected phase voltage which is an estimate of the voltage actually needed to produce a correct phase shift for a particular phase shift element.
Further, the drive voltages for all elements are corrected by multiplication against the same correction factor, thereby limiting system accuracy. This accuracy limitation results from the fact that temperature-based phase correction is dependent on drive voltage amplitude, whereas the prior system operates with a presumption that temperature-based phase correction is independent of drive voltage amplitude.
A need thus exists to imrove upon the current state of the pertaining art by providing better compensation for environmental parameters in the beam steering of a radar system having a ferroelectric phased array antenna.